Development of Soft Underwater Robot that Mimics Red Muscle and Tendon Structure of Fish

Fish robots are attracting attention due to their highly efficient propulsion performance, adaptation to multiple motions, and low negative impact on the environment. In recent years, the development of soft underwater robots has been gaining attention because of their simple structure and compact size. However, it is difficult to reproduce the various motions of fish using a soft body. We propose a hybrid robot with a rigid multi-linked structure and antagonistically placed soft actuators, focusing on the fish anatomical structure which is composed of rigid vertebrae and soft muscles and tendons. In this paper, we focused on the red muscles, which are believed to play important role in the steady swimming of fish, and created a robot that mimics its muscle-tendon structure. First, we designed the module based on the red muscles and the tendons around them and investigated the geometric characteristics. Then, a prototype of the robot was fabricated by connecting the modules, and the performance of the robot was tested in the air. From the experiment, the generation of undulatory motion in which the amplitude increases posteriorly at various speeds of undulation waves was confirmed. Finally, a swimming experiment was conducted by attaching an arrangement of plates that resembles the body shape of fish, and it was confirmed that it is propelled at the maximum swimming speed of 19.6 mm/s by the generated motion while the swimming speed depends on the speed of the undulation waves.